The utilization of collimated ion beams to expose certain known and commercially available polymer (resist) materials is generally known in the art of ion beam lithography and is disclosed, for example, in an article entitled "Focused Ion Beams in Microfabrication", R. L. Seliger and W. P. Fleming in The Journal of Applied Physics, Vol. 45, No. 3, March 1974. In many wafer processing operations, ion beam lithography is preferred over electron beam lithography and photolithography because of the reduced resist exposure times made possible by the use of ion beams.
To achieve optimum resolution during the selective ion implantation of certain ion species into layers of resist, care should be taken to minimize the scattering or dispersion of the ion beams as they are selectively masked in the ion beam path between the source of ions and the supporting wafer or substrate upon which the ion beam resist is deposited. Thus, by minimizing the scatter angle of the ion beam during its passage through selective masking materials, one can correspondingly minimize certain device line widths, such as the gate width (length, Lg,) on a high frequency field effect transistor.
The minimization of the scatter angle (.theta.) of transmitted ion beams is particularly important when use is made of off-contact lithographic processes in which there is a space, S, between the transmission mask and the target resist. For a given value of .theta., the magnitude of the lateral deflection observed at the target increases with an increase in the space S. However, it is desirable in an off-contact process to have a significant space S between the mask and the target in order to prevent the mask from becoming contaminated, which in turn, allows multiple use of the same mask without cleaning after each use. In addition, it is advantageous to use an off-contact process because it provides a large area exposure technique which is often necessary to satisfy large-scale production requirements. Thus, it is desirable to use an off-contact process having the maximum attainable distance between the mask and the target resist, while at the same time minimizing the scatter angle of the ions transmitted through the mask, thereby producing a well-replicated, high resolution resist pattern. Transistor gate lengths of 0.5 micrometers are now desirable, but not yet attainable, in many X-band and higher frequency field effect transistors. Thus, the desirability of even further improving the above ion beam resolution and minimizing ion scattering is manifest.
One technique which has been used successfully to improve these above characteristics is disclosed and claimed in my allowed copending application, Ser. No. 794,288, assigned to the present assignee. In this copending application, a hyperthin taut amorphous membrane of a material such as aluminum oxide (Al.sub.2 O.sub.3) is used as the support member for an ion absorption mask. The mask is aligned with a target substrate having a layer of resist thereon, and ions are projected onto the mask and are transmitted through selected area of the hyperthin Al.sub.2 O.sub.3 membrane which are exposed by patterned openings in the ion absorption material. The transmitted ions strike the resist-covered substrate and thereby expose predefined areas therein. Using this process, the accelerated ions have a very minimum of mask particles to pass through relative to prior art processes. Consequently, this approach provides substantial improvements in minimizing ion scattering and thus improving resolution in the art of ion beam lithography.